AROMATIC REACTIVITY .38. PROTODESILYLATION OF 1,2-DIHYDROBENZOCYCLOBUTENE INDAN AND TETRALIN

Rates of cleavage (protodesilylation) of some trimethylsilyl derivatives of reduced benzocycloalkenes and o-xylene by a mixture of 10.6M-aqueous perchloric acid and methanol (2:5 v/v) have been measured spectrophotometrically. Values (min.-1) of the first-order rate constants (x 103) at 50.0° are as follows: 3-trimethylsilyl-o-xylene, 193; 4-trimethysilyl-o- xylene, 153; 1,2-dihydro-3-trimethylsilylbenzocyclobutene, 16.1; 1,2-dihydro-4-trimethylsilylbenzocyclobutene, 154; 4-trimethylsilylindan, 77; 5-trimethylsilylindan, 210; 5-trimethylsilyltetralin, 235; 6- trimethylsilyltetralin, 181. (Ring-opening, which complicates most studies of the behaviour of 1,2-dihydrobenzocyclobutene and its derivatives towards electrophilic reagents, does not occur under the cleavage conditions.) The main features of the results are the low reactivity of the 3-(aryl α-) and the normal reactivity of the 4- (aryl β-) position of 1,2- dihydrobenzocyclobutene in comparison with the corresponding positions of o-xylene. The aryl α-position of indan shows a somewhat lower reactivity, but the aryl β-position shows a slightly enhanced reactivity, as do both the aryl α- and β-positions of tetralin. It is suggested that strain and carbon-hydridization effects of the small rings operate jointly to determine the reactivities. Use of desilylation is suggested as a possible route to 3-substituted 1,2-dihydrobenzocyclobutenes.